Polyethylene polymers are, under normal conditions, semicrystalline polymers containing crystalline regions interspersed with amorphous regions. In particular, polyethylene polymers crystallize by folding of the polyethylene chain, which produces crystalline lamellae interspersed with an amorphous polyethylene phase. When polyethylene polymers are processed under conditions that subject the molten polymer to relatively little strain, the polymer chains in the polymer melt are relaxed in a random coil configuration. In the absence of heterogeneous nuclei (impurities or intentionally added agents), the polymer melt (e.g., polyethylene melt) cools until sufficient inter-chain interaction occurs to spontaneously initiate chain-folding and subsequent crystalline lamellar growth. This lamellar growth is typically spherulitic and exhibits very little, if any, preferred orientation of the polyethylene's crystallographic a, b, and c axes in three dimensions.
In extensional strain processes like blown film, however, and depending on the exact strain level, a greater or lesser degree of the melt chains may be extended in the flow direction. Alignment and attraction of some of these extended chains can lead to crystallization of fibrils which form at higher temperatures than the bulk of primary crystallization and are oriented in the flow direction. These fibrils can be very effective sites for further nucleation, with the subsequent direction of fastest growth (b axis of the polyethylene orthorhombic unit cell) normal to the fibril length. The b axis (or lamellar fast growth direction) is distributed more or less radially around these fibrils (and therefore the b-axis is normal to the flow direction). This morphology is referred to as a row nucleated, or “shish-kebab” morphology, with the fibrils forming the “shishes” and the chain-folded lamellae forming the “kebabs” growing normal to the “shishes.” The degree of extensional strain and the potential degree of relaxation of the melt determine the exact final morphology. At intermediate strain and/or with moderate relaxation possible, i.e. with moderate trapped flow direction orientation approaching the crystallization temperature, fibril nucleation density is moderate. Based on widely accepted literature models, Keller/Machin 1 (KM 1) morphology results, wherein the b axis is primarily normal to the fibrils and distributed radially around fibrils (and therefore the b-axis is normal to the flow direction), and the a axis shows at least some net level of orientation parallel to fibril or flow direction. Under more extreme combinations of extensional strain and lack of melt relaxation, fibril nucleation density is relatively higher. Keller/Machin 2 (KM 2) morphology results, wherein b axis orientation shows strong net orientation normal to fibril length and distributed radially around the fibril (and therefore the b-axis is normal to the flow direction). Lamellar twisting is not possible due to very high fibril nucleation density, and the c axis shows a significant net orientation parallel to fibril or flow direction.
The KM1 and KM2 morphologies can lead to certain undesirable properties in an article. For example, polyethylene films exhibiting either the KM1 or KM2 morphology exhibit unbalanced tear strength between the machine direction and the transverse direction. While this lack of balance may not be problematic for certain articles and applications, it can prove troublesome for tearable films. Tearable films that exhibit a lack of balance in tear strength can have a tear that suddenly changes the direction in which it propagates through the film. This can be particularly problematic for tearable films used in food packaging, where a controlled tear is desired in order to avoid spilling the contents of the packaging.
While the addition of nucleating agents can change certain aspects of the crystallization, their addition has not yet been observed to produce polyethylene in which the b axis of the orthorhombic polyethylene unit cell is preferentially parallel to the machine direction of the polyethylene article. Applicants believe that such a morphology is desirable and will enable one to produce polyethylene articles having unique physical properties, such as more balanced tear strength in the machine and transverse directions, higher machine direction stiffness, better barrier, and higher heat distortion temperature (HDT) as well as other beneficial properties.